Condensers for alternating current and high current applications are constituted of two dielectric foils, one of which is provided from a contact edge to shortly before the opposite edge with a metal layer which can be vapor deposited and can be composed of an alloy of aluminum and zinc. Such condensers are extremely compact and because of their small volume are widely used.
When the principal criterion is ease of deposition of the metalic layer, it is generally composed of zinc. When the principal criterion is low electrical resistance, the metal layer is usually composed of aluminum.
It has also been found that long term storage and long life points to the use of zinc-coated foils and that, after fabrication of the condenser, upon sealing failures or the application of high field strengths, changes in the metal layer can occur. For example, the highly conductive aluminum may be transformed into practically insulating aluminum oxide by oxidation, thereby reducing the electrode surface area and reducing the capacity of the condenser.
An increase in foil thickness tends to reduce the mean field strength correspondingly and to reduce the oxidation and hydroxylation of the metallic coatings. However, since the oxidation and hydroxylation processes are concentrated primarily along the edges and may be promoted by the presence of dust or like substances which increase the field strength in these regions, an increase in the thickness does little to overcome the problem of storage deterioration and the like. The reduction in field strength also reduces the selfhealing capability of burning away such bridges.
Consequently, attention has been directed to zinc as a perfect metal for such capacitors. While this metal has a higher corrosion reistance than aluminum, however, its reduced electrical conductivity requires it to be vapor deposited in greater thicknesses which facilitate selfhealing. A disadvantage is the fact that the corrosion products of zinc have substantially higher volume than the starting layer and the formation of corrosion products gives rise to mechanical deterioration of the capacitor.
Faced with these problems, a host of improvements have been proposed.
For example, it has been proposed to provide an initial layer of chromium, silver, titanium or alloys thereof upon which a second thicker layer of aluminum can be applied (German patent document Printed Application DE-AS 23 59 432). From German patent document Open Application DE-OS 24 11 813, it is known to provide a polyscrystalline insulating layer under an aluminum coating while German patent document Open Application DE-OS 26 41 232 describes the stabilization of conductive layers of aluminum, zinc and/or cadmium with metal compounds, for example, oxides. In German patent document Open Application DE-OS 27 00 013, a first comparatively thick layer of aluminum is coated with a second comparatively thin layer of copper, nickel, cobalt, titanium, tungsten or zinc. These approaches are all designed to improve the corrosion resistance of aluminum without inordinately increasing the thickness of the metal layers and thus to reduce the capacity loss of condensers.
German patent documents Open Application DE-OS 27 03 636, DE-OS 27 30 038 and DE-OS 29 02 195 describe electrode materials in which metal layers of alloys are provided, the alloys generally containing 15 to 18 atomic percent aluminum, and copper, zinc, manganese, tin, silver, chromium, iron and lead as other components. They also contemplate the use of aluminum alloys which can contain 0.5 to 10%, preferably 2 to 5% copper and even intermetallic compounds of aluminum, nickel, magnesium, titanium, hafnium, beryllium or bismuth. Even with these various aluminum containing products, however, it is not possible to solve completely the loss of capacity of condensers fabricated from vapor deposited metal foil or to insure constant capacity under loading.